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Electric Circuits AC Series Circuits Presentation
1. Course Title: Electric Circuits
(ECI:22324)
Presentation on Unit 1: AC series circuits
On Topics
1.1 Generation of alternating voltage, phasor representation of
sinusoidal quantities
2. 1.1 Generation of alternating voltage and currents
οAn alternating voltage may be generated:
1. By rotating a coil at constant angular velocity in a uniform magnetic field.
2. By rotating a magnetic field at a constant angular velocity within a
stationary coil.
3.
4. Continueβ¦.
Consider a rectangular coil of n turns rotating in anticlockwise direction with an
angular velocity of w rad/sec in uniform magnetic field.
β’ Let the time be measured from the instant the plane of the coil coincide with
OX-axis. In this position of the coil, the flux linking with the coil has its maximum
value max. Fig.1.2 (i)
β’ Let the coil turn through an angle ΞΈ(=Οt) in anticlockwise direction in t
seconds and assume the position shown in fig 1.2(ii)flux .
β’ In this position, the maximum flux max acting vertically downward can
be resolved into two perpendicular components:
β’ i) Component max sinΟt parallel to the plane the coil. This component
induces no emf in the coil.
β’ ii) Component max cosΟt perpendicular to the plane of the coil. This
component induces emf in the coil.
5. Continueβ¦
β’ Flux linkage of the coil at the considered instant (i.e. at ΞΈ angle)
=No. of turns * flux linking
= n max cosΟt β¦β¦β¦β¦β¦β¦β¦β¦β¦.β¦β¦β¦β¦.(1)
Fig.
6. Continueβ¦.
β’ According to Faradayβs law of electromagnetic induction,
the emf induced in coil is equal to the rate of change of flux linkage of the coil.
Hence emf v at the considered instant is given by,
β΄ π£ = β
π
ππ‘
(π β max cosππ‘)
β΄ π£ = βπβ maxπ(βsinππ‘)
β΄ π£ = ππβ max(sinππ‘) β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦(2)
Value of π£ is maximum, when sinΟt=1
β΄ ππ =
πβ maxΟ
From equation (2),
β΄ π£ = Vm(π ππΟt) β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦.(3)
From equation (3),
β’ If a coil rotating with a constant angular velocity in a uniform magnetic field
produces a sinusoidal alternating emf.
Similarly, equation of the alternating current is given by,
β΄ π = ImsinΟt β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦β¦...(4)
7. 1.12 Important AC Terminology
1.) Waveform: The shape of the curve obtained by plotting the instantaneous value of
voltage or current as ordinate against time is called waveform.
2.) Instantaneous Value: The value of alternating quantity at any instant is called
instantaneous value. The instantaneous value of alternating voltage and current are
represented by π£ and π .
3.) Cycle: One complete set of positive and negative values of an alternating quantity is
known as cycle.
8. Continueβ¦.
4.) Alteration: One half cycle of an alternating quantity is called alteration.
An alteration 180 degree electrical.
5.) Time period: The time taken in seconds to complete one cycle of an
alternating quantity is called its time period. It is generally represented by
π.
6.) Frequency: The number of cycle complete in one seconds is called the
frequency (π)of the alternating quantity. It is measured in cycle/sec or
hertz. One hertz is equal to 1cycles/seconds.
7.) Amplitude: The maximum value attained by an alternating quantity is
called its amplitude or peak value. The amplitude of an alternating voltage
or current is designated by π
π or πΌπ respectively.
9. 1.13 Important Relations
οTime period and frequency:
β’ Consider an alternating quantity having a frequency of f c/s (Hz) and time period T sec.
β’ Time taken to complete f cycle=1 seconds
β’ Time taken to complete 1 cycle= 1/f second.
β’ But the time taken to complete one cycle is the time period T
β’ π =
1
π
ππ π =
1
π
οAngular velocity and frequency:
β’ In a one revolution of the coil the angle turned is 2 radian and voltage wave cycle
complete one cycle.
β’ π΄πππ’πππ πππππππ‘π¦ =
π΄ππππ π‘π’ππππ
ππππ π‘ππππ
β’ π =
2π
π
β’ π = 2π β
1
π
β’ π = 2ππ
10. 1.14 Values of alternating voltage and current
i. Peak Value
ii. Average value or mean value
iii. RMS value(root mean square) or effective value
iv. Peak to peak value
11. Continue..
i. Peak Value:
β’ It is maximum value attained by alternating quantity.
β’ It is represented by π
π and πΌπ of alternating voltage and current
respectively.
β’ Peak value is important in case of testing of material.
β’ Peak value is not used to specify the magnitude of alternating voltage or
current.
12. Continueβ¦
ii. Average value:
β’ The average value of alternating current is zero over one cycle because
positive area exactly cancels the negative area.
β’ However, half cycles average value is not zero.
β’ Therefore , whenever the average value of alternating current or voltage
is asked, it is understood for half cycles.
13. Continueβ¦
Expression for alternating current is
π = π°πππππ½ β¦β¦β¦β¦β¦ΞΈ=Οt
β’ Average value of current is given by
π°ππ =
ππππ ππ ππππ πππππ
ππππ ππππππ ππ ππππ πππππ
π°ππ = π
π
ππ π½
π
=
π
π π
π
π°πππππ½π π½
=
πΌπ
π
βπππ π 0
π
=
πΌπ
π
βπππ π β (βπππ 0)
=
πΌπ
π
β(β1) β (β 1 )
15. Continueβ¦
iii. RMS (root mean square) value or effective value:
β’ The equation of the alternating current varying sinusoidally is given by,
π = πΌππ πππ
β’ Consider an elementary strip of thickness ππ in first half cycle of the
squared current wave.
β’ let π2 be the mid ordinate of the strip
β’ Therefore, ππππ ππ πππ πππππ = ππ
π π½
Area of half squared wave = π
π
πππ π½
= π
π
π°π
ππππππ½π π½
π
17. Continueβ¦
iv. Peak to Peak value:
(1) ππππ ππππππ =
πΉπ΄πΊ πππππ
π¨ππππππ πππππ
=
π.πππβπ¦ππ± πππππ
π.πππβπππ.πππππ
= π. ππ
(2) Peak ππππππ =
πππ. πππππ
πΉπ΄πΊ πππππ
=
π¦ππ± πππππ
π.πππβπππ.πππππ
= π. πππ
β’ The peak factor is great important because it indicates the maximum value
voltage being applied to the various of the apparatus.
18. Continueβ¦
β’ Phase and phase difference:
οPhase: phase of particular value of an alternating quantity is the
fractional part of time period or cycle through which the quantity has
advance from the selected zero position or reference.
οPhase difference refers to the angular displacement between different
waveforms of the same frequency.
20. Phasor representation of alternating quantities
β’ When number of waveforms are drawn in the same figure, the complexity
of diagram increases and it becomes very difficult to extract the information
from the waveforms. Therefore, to extract the same information, simplified
alternate approach is preferred, called βPhasor representation of Sinusoidal
quantityβ.
β’ A sinusoidal quantity is represented by a rotating vector or rotating phasor
βAβ whose length is equal to the amplitude of the quantity βAmβ, as shown
above. The points on the waveform are represented by the positions of the
phasor during rotation drawn from the same reference point. The phasor
making an angle of ππ‘ with respect to positive x-axis reference, represents
the instantaneous value of the quantity at an angle of ππ‘ from its zero
value, as shown above. In fact, the vertical component of the phasor
represents the magnitude of the quantity at that particular instant. From
the above diagram, it is clear that the vertical component of the phasor is
βAm sin(ππ‘ )β which is the instantaneous value of the quantity at instant βππ‘
β.
β’ The speed of rotation of the phasor is equal to w rad/sec where π = 2Οf.
β’ One rotation of the phasor corresponds to one cycle of the alternating
waveform as shown in figure.
21. Numerical
Q.1. An alternating current is given by π = πππ. ππππππππ.Find:(1) The maximum
value (2) frequency (3) Time period (4) The instantaneous value when π is πππ.
Solution: Given equation: π = 141.4π ππ314t
comparing given equation of alternating current with standard form,π =
πΌππ ππππ‘
We get, πΌπ = 141.4 , π = 314
1) Maximum value π°π = πππ. π π΄πππ
2) Frequency , π = 2ππ β΄ π =
π
2π
=
314
2π
= ππ π―π
3) Time period, T =
1
π
β΄ π =
1
50
= π. ππ πππ
4) Instantaneous value when π‘ = 3ππ
π = 141.4π ππ314π‘
= 141.4π ππ314 β 3 β
10β3 β¦ β¦ β¦ . . (πππ‘π: π’π π π πππππ‘ππππ ππππ’πππ‘ππ ππ π ππ ππππ)
π = πππ. ππ π¨πππ
22. Numerical
Q.2. Express:
i. π = ππβ πππ in rectangular form
ii. π = ππ + ππ in polar form
Solution: Using scientific calculator,
i. π = 10β 600 = π + π£π. ππ β¦
ii. π = 16 + π8 = ππ. ππβ ππ. πππ β¦
23. Numerical
Q.3. calculate frequency ,rms value, average value and amplitude of the waveform shown in fig.
οSolution: from fig. we have ππππ ππππππ , π = 20ππ
1. πππππ’ππππ¦, π =
1
π
β΄ π =
1
20β10β3 = ππ π―π
2. π΄πππππ‘π’ππ = ππππ π£πππ’π = πππ π½ π½π
3. π
πππ = π =
ππ
2
=
100
2
= ππ. ππ πππππ.
4. π
ππ£πππππ =
2ππ
π
= 0.637 β π
π = ππ. π π½
100 π
π
20ππ